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  typescript类型断言解决一些常见问题
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      <a href="/2020/08/13/typescript%E7%B1%BB%E5%9E%8B%E6%96%AD%E8%A8%80%E8%A7%A3%E5%86%B3%E4%B8%80%E4%BA%9B%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98/" class="article-date">
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  <p>最近在维护一个vue+ts项目的过程中遇到了一些typescript类型报错，简单来讲就是ts在进行类型推断时出现了一些错误，需要我们选择性的使用类型断言来解决报错，下面是我收集到的目前出现错误的情况，后续如果有发现新问题会持续更新。</p>
<h1 id="Property-‘insertRule’-does-not-exist-on-type-‘StyleSheet’"><a href="#Property-‘insertRule’-does-not-exist-on-type-‘StyleSheet’" class="headerlink" title="Property  ‘insertRule’ does not exist on type ‘StyleSheet’"></a>Property  ‘insertRule’ does not exist on type ‘StyleSheet’</h1><p>这个是我在Vue中开发一个CSS动画时候报的错，具体在下面这段代码中：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> style = <span class="built_in">document</span>.createElement(<span class="string">'style'</span>);</span><br><span class="line">style.setAttribute(<span class="string">'type'</span>, <span class="string">'text/css'</span>);</span><br><span class="line"><span class="built_in">document</span>.head.appendChild(style);</span><br><span class="line"><span class="keyword">const</span> sheet = style.sheet;</span><br><span class="line"></span><br><span class="line">sheet.insertRule(</span><br><span class="line">    <span class="string">`@keyframes roller &#123;</span></span><br><span class="line"><span class="string">        from &#123;</span></span><br><span class="line"><span class="string">            top: 0px;</span></span><br><span class="line"><span class="string">        &#125;</span></span><br><span class="line"><span class="string">        to &#123;</span></span><br><span class="line"><span class="string">            top: -<span class="subst">$&#123;(<span class="keyword">this</span>.prizeArr.length - <span class="number">1</span>) * <span class="keyword">this</span>.itemHeight&#125;</span>px;</span></span><br><span class="line"><span class="string">        &#125;</span></span><br><span class="line"><span class="string">    &#125;`</span>, <span class="number">0</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure>
<a id="more"></a>
<p>业务需求是动态地往html文档头部增加一个style标签，写入对应的animation的keyframe规则，但是如果像上面这样就会报两个错：<br><img src="https://img-blog.csdnimg.cn/20200813113523652.png#pic_center" alt=""><br><img src="https://img-blog.csdnimg.cn/2020081311353553.png#pic_center" alt=""></p>
<p>首先看第一个问题，Object is possible null，也就是当我们申明sheet变量时没给它一个确认的类型，那么ts就会进行推断，推断的结果就是sheet是一个<code>null  |  StyleSheet</code>类型，这个时候调用insertRule方法就会发生类型错误</p>
<p>这个时候当我们确定sheet不会是null时，就可以使用类型断言：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">(sheet <span class="keyword">as</span> StyleSheet).insertRule(</span><br><span class="line"><span class="string">`@keyframes roller &#123;</span></span><br><span class="line"><span class="string">       from &#123;</span></span><br><span class="line"><span class="string">           top: 0px;</span></span><br><span class="line"><span class="string">       &#125;</span></span><br><span class="line"><span class="string">       to &#123;</span></span><br><span class="line"><span class="string">           top: -<span class="subst">$&#123;(<span class="keyword">this</span>.prizeArr.length - <span class="number">1</span>) * <span class="keyword">this</span>.itemHeight&#125;</span>px;</span></span><br><span class="line"><span class="string">       &#125;</span></span><br><span class="line"><span class="string">   &#125;`</span>, <span class="number">0</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure>
<p>那么这个时候就会出现第二个错：</p>
<blockquote>
<p>Property ‘insertRule’ does not exist on type ‘StyleSheet’</p>
</blockquote>
<p>这是因为类型StyleSheet确实缺少addRule和insertRule方法。这些方法是在CSSStyleSheet类型上定义的。</p>
<p>只需要这样修改就可以了：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">(sheet <span class="keyword">as</span> CSSStyleSheet).insertRule(</span><br><span class="line"><span class="string">`@keyframes roller &#123;</span></span><br><span class="line"><span class="string">       from &#123;</span></span><br><span class="line"><span class="string">           top: 0px;</span></span><br><span class="line"><span class="string">       &#125;</span></span><br><span class="line"><span class="string">       to &#123;</span></span><br><span class="line"><span class="string">           top: -<span class="subst">$&#123;(<span class="keyword">this</span>.prizeArr.length - <span class="number">1</span>) * <span class="keyword">this</span>.itemHeight&#125;</span>px;</span></span><br><span class="line"><span class="string">       &#125;</span></span><br><span class="line"><span class="string">   &#125;`</span>, <span class="number">0</span></span><br><span class="line">)</span><br></pre></td></tr></table></figure>
<h1 id="Object-is-possibly-‘null’-for-regex"><a href="#Object-is-possibly-‘null’-for-regex" class="headerlink" title="Object is possibly ‘null’ for regex"></a>Object is possibly ‘null’ for regex</h1><p>这是在使用正则表达式时会发生的一个错，同样也是因为ts类型推断结果里面包含了null导致不能正确调用变量的方法。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> body = <span class="regexp">/&lt;body.*?&gt;([\s\S]*)&lt;\/body&gt;/</span>.exec(html)[<span class="number">1</span>];</span><br></pre></td></tr></table></figure>
<p>这个时候可以使用非空类型断言来解决：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> body = <span class="regexp">/&lt;body.*?&gt;([\s\S]*)&lt;\/body&gt;/</span>.exec(html)![<span class="number">1</span>];</span><br></pre></td></tr></table></figure>
<h1 id="Property-‘validate’-does-not-exist-on-type-‘Vue-Element-Vue-Element-’"><a href="#Property-‘validate’-does-not-exist-on-type-‘Vue-Element-Vue-Element-’" class="headerlink" title="Property ‘validate’ does not exist on type ‘Vue | Element | Vue[] | Element[]’."></a>Property ‘validate’ does not exist on type ‘Vue | Element | Vue[] | Element[]’.</h1><p>这是一个在vue中会报的错，我们在父组件调用子组件中的方法时，要么使用emit，要么直接调用，就像下面这样</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 父组件</span></span><br><span class="line">&lt;template&gt;</span><br><span class="line">	&lt;son ref=<span class="string">"sonEle"</span>/&gt;</span><br><span class="line">&lt;<span class="regexp">/template&gt;</span></span><br><span class="line"><span class="regexp"></span></span><br><span class="line"><span class="regexp">&lt;script&gt;</span></span><br><span class="line"><span class="regexp">/</span><span class="regexp">/ 此处省略一些引入的文件</span></span><br><span class="line"><span class="regexp">@Component&#123;&#125;</span></span><br><span class="line"><span class="regexp">export default class Parent extends Vue &#123;</span></span><br><span class="line"><span class="regexp">	created() &#123;</span></span><br><span class="line"><span class="regexp">		this.$refs.sonEle.play();</span></span><br><span class="line"><span class="regexp">	&#125;</span></span><br><span class="line"><span class="regexp">&#125; </span></span><br><span class="line"><span class="regexp">&lt;/</span>script&gt;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 子组件</span></span><br><span class="line">&lt;template&gt;</span><br><span class="line">	<span class="comment">// ...</span></span><br><span class="line">&lt;<span class="regexp">/template&gt;</span></span><br><span class="line"><span class="regexp"></span></span><br><span class="line"><span class="regexp">&lt;script&gt;</span></span><br><span class="line"><span class="regexp">/</span><span class="regexp">/ 此处省略一些引入的文件</span></span><br><span class="line"><span class="regexp">@Component&#123;&#125;</span></span><br><span class="line"><span class="regexp">export default class Parent extends Vue &#123;</span></span><br><span class="line"><span class="regexp">	play() &#123;</span></span><br><span class="line"><span class="regexp">		/</span><span class="regexp">/ ...</span></span><br><span class="line"><span class="regexp">	&#125;</span></span><br><span class="line"><span class="regexp">&#125; </span></span><br><span class="line"><span class="regexp">&lt;/</span>script&gt;</span><br></pre></td></tr></table></figure>
<p>这个时候就会发生ts的类型报错：Property ‘play’ does not exist on type ‘Vue | Element | Vue[] | Element[]’.</p>
<p>同样使用类型断言：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">(<span class="keyword">this</span>.$refs.sonEle <span class="keyword">as</span> Vue &amp; &#123; <span class="attr">play</span>: <span class="function"><span class="params">()</span> =&gt;</span> <span class="literal">null</span> &#125;).play();</span><br></pre></td></tr></table></figure><br>如果这个方法在组建内被多次使用，可以使用这个方法：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">computed: &#123;</span><br><span class="line">  play(): Vue &amp; &#123; <span class="attr">play</span>: <span class="function"><span class="params">()</span> =&gt;</span> <span class="literal">null</span> &#125; &#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span>.$refs.sonEle <span class="keyword">as</span> Vue &amp; &#123; <span class="attr">play</span>: <span class="function"><span class="params">()</span> =&gt;</span> <span class="literal">null</span> &#125;;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
 
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  <p class="reward-p"><i class="ri-cup-line"></i>请我喝杯咖啡吧~</p>
  <div class="reward-box">
    
    <div class="reward-item">
      <img class="reward-img" src="/images/alipay.png">
      <span class="reward-type">支付宝</span>
    </div>
    
    
    <div class="reward-item">
      <img class="reward-img" src="/images/wxpay.png">
      <span class="reward-type">微信</span>
    </div>
    
  </div>
</div>
    
<script src="/js/jquery-2.0.3.min.js"></script>


<script src="/js/lazyload.min.js"></script>

<!-- Tocbot -->


<script src="/js/tocbot.min.js"></script>

<script>
  tocbot.init({
    tocSelector: '.tocbot',
    contentSelector: '.article-entry',
    headingSelector: 'h1, h2, h3, h4, h5, h6',
    hasInnerContainers: true,
    scrollSmooth: true,
    scrollContainer: 'main',
    positionFixedSelector: '.tocbot',
    positionFixedClass: 'is-position-fixed',
    fixedSidebarOffset: 'auto'
  });
</script>

<script src="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.js"></script>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.css">
<script src="https://cdn.jsdelivr.net/npm/justifiedGallery@3.7.0/dist/js/jquery.justifiedGallery.min.js"></script>

<script src="/dist/main.js"></script>

<!-- ImageViewer -->

<!-- Root element of PhotoSwipe. Must have class pswp. -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">

    <!-- Background of PhotoSwipe. 
         It's a separate element as animating opacity is faster than rgba(). -->
    <div class="pswp__bg"></div>

    <!-- Slides wrapper with overflow:hidden. -->
    <div class="pswp__scroll-wrap">

        <!-- Container that holds slides. 
            PhotoSwipe keeps only 3 of them in the DOM to save memory.
            Don't modify these 3 pswp__item elements, data is added later on. -->
        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>

<!-- MathJax -->

<!-- Katex -->

<!-- busuanzi  -->


<script src="/js/busuanzi-2.3.pure.min.js"></script>


<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->


<link rel="stylesheet" href="/css/clipboard.css">

<script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>


<!-- CanvasBackground -->


    
  </div>
</body>

</html>